Unusual thermo(photo)chromic properties of some mononitro- and dinitro- substituted 3′-alkyl indolospirobenzopyrans

Isomeric equilbria of dinitro-substituted indolospirobenzopyrans, possessing 3'-gem-methyl- or 3'-cyclohexyl-substitutents, have been investigated using H-1 NMR spectroscopy at six temperatures, ranging from 298 K to 410 K; isomerisation processes in a methanolic solution have been monitored by spectrophotometry. For the mononitro-substituted compounds, equilibrium favoured the colourless spirocyclic isomers. Reversion of the coloured merocyanine isomers, generated by UV irradiation, followed first-order kinetics. For the dinitro-substituted compounds the coloured merocyanine isomers predominated. Following decolourisation by visible light photoirradiation, reversion towards the merocyanine structures were particularly slow, absorbances unusually increasing sigmoidally. UV. spectral observations for the gem-methyl- 1 and 3'-cyclohexyl-substituted systems 2 are qualitatively consistent with the simultaneous involvement of two relatively slow rate determining steps, possessing slowly forming and long-lived intermediates - postulated to be the oxygen protonated pyran-ring of the spirocyclic structure and TCC merocyanine isomer, the latter undergoing relatively slow isomerisation about the central beta-alkenic bond to the TTC isomer. (C) 2015 Elsevier Ltd. All rights reserved.